A person who doubts himself is like a man who would enlist in the ranks of his enemies and bear arms agains himself. He makes his failure certain by himself being the first person to be convinced of it.
Who never doubted, never half believed. Where doubt is, there truth is - it is her shadow.
When you doubt, abstain.
Doubt, indulged and cherished, is in danger of becoming denial; but if honest, and bent on thorough investigation, it may soon lead to full establishment of the truth.
Men become civilized, not in proportion to their willingness to believe, but in proportion to their readiness to doubt.
Doubt begins only at the last frontiers of what is possible.
Doubt is the father of invention.